1. Field of the Invention
This invention generally relates to a facsimile machine, and, in particular, to a facsimile machine having a capability of adjusting the level of a signal to be transmitted.
2. Description of the Prior Art
In general, a signal to be transmitted to a facsimile receiver from a facsimile transmitter is desired to have a constant transmission level irrespective of the frequency of a signal to be transmitted. In addition, due to the recent rapid prevalence of facsimile machines over the world, transmission is often made not only to a facsimile receiver in one country but also to a facsimile machine in a foreign country. However, since the regulated value of a signal transmission level generally differs from country to country, a facsimile machine is required to absorb such differences in the signal transmission level so as to allow to carry out transmission of a signal smoothly without regard to the location of the facsimile receiver.
Signals to be supplied to a network from a facsimile machine include an image signal and a control signal, which are output through a MODEM of the facsimile machine, and also a DTMF (Dual Tone Multi-Frequency) signal, which is output from a DTMF signal generating circuit when a push button of a push phone has been depressed. Such MODEM signals (i.e., image signal and control signal) and DTMF signal to be output to a network from a facsimile machine are regulated in their levels in each country or a region of the world. However, there is typically a scatter in the transmission level in each of these signals output from a facsimile machine. That is,
(1) Each of these signals has a unique frequency, but there is a slight scatter in the output level of a signal to be output from a MODEM according to frequency; and
(2) There is a specific frequency characteristic in each of the circuits between the output terminal of a MODEM and the output terminal of a facsimile machine and between the DTMF signal generating circuit and the output terminal of a facsimile machine.
In addition to a scatter in the transmission level of each of the signals output from a facsimile machine as described above, the regulated value of a transmission level of a signal output to a network from a facsimile machine often differs from country to country. Under the circumstances, in a prior art technique, it is typically so structured that all of the signals output from a facsimile machine are limited to be equal to or lower than a specified level uniformly by a transmission signal level adjusting attenuator in a fixed manner. In such a prior art structure, since all of the signals output from a facsimile machine are regulated to be equal to or less than a specified level uniformly, it is impossible to attenuate the output level in accordance with the frequency of each signal. As a result, there is produced a scatter in the frequency of each signal depending on the circuit characteristic, so that there is a chance that the regulated value is exceeded depending on the frequency and there is also a chance that a sufficient output level is not obtained because the attenuation becomes excessive.
In order to cope with this problem, it is conceivable to provide an attenuator having a differing rate of attenuation depending on frequency so as to make all of the signals fall within a regulated range. In this case, however, it is required to provide a circuit complicated in structure. Besides, when the circuit characteristic of a MODEM or a DTMF signal generating circuit varies for some reason, complicated adjustments for absorbing such variations must be carried out, which tend to push up the cost. In addition, in such a hardware structure, it is necessary to prepare individual adjusting circuits for satisfying the various regulated values for a signal transmission level in various countries, which tend to make the machine complicated in structure and also to push up the cost.